Selection, diversity combining or satellite mimo to mitigate scintillation and/or near-terrestrial multipath to user devices

ABSTRACT

A ground station processes downlink signals received from respective satellites. The ground station has a plurality of signal conditioning devices each receiving a respective one of the downlink signals and providing a conditioned downlink signal. A plurality of Doppler and/or Delay compensator devices each receive a respective conditioned downlink signal from a respective one of the plurality of signal conditioning devices. The compensator devices conduct Doppler and/or Delay compensation on the received conditioned downlink signal, and provide a compensated downlink signal output. A selector or diversity combiner receives the compensated downlink signal from each of the plurality of Doppler and/or Delay compensators. The selector or diversity combiner selects one of the received compensated downlink signals based on received signal strength of each received compensated downlink signal to provide a selected downlink signal, or diversity combines all of the received compensated downlink signals to provide a diversity combined signal. The selector or diversity combiner provides the selected downlink signal or the diversity combined signal to an eNodeB.

RELATED APPLICATIONS

This application claims the benefit of priority of India ApplicationNos. 201911025299, filed Jun. 25, 2019, India Application No.201911026070, filed Jun. 29, 2019, and U.S. Provisional Application No.62/951,618, filed on Dec. 20, 2019, the content of which is relied uponand incorporated herein by reference in its entirety.

BACKGROUND

The present disclosure relates to standard mobile user equipment (UEs)to be connected to base-station equipment (e.g., eNodeB's or gNodeB's in4G and 5G mobile communications parlance) located at gateways, with atleast two directional antennas, tracking Low-Earth Orbit (LEO) relaysatellites. The communications between the UEs and the LEO satellitesare typically in UHF/L-band (i.e., 600-900 MHz or 1800-2100 MHz bands),while the satellite-to-gateway links are in Q/V band. Impairments ofinterest in the UHF/L-band to LEO satellite links are ionosphericscintillation and terrestrial multipath losses. The primary impairmentsin the Q/V band link from/to the gateway to/from the satellite arerain-induced attenuation and/or depolarization. While diversitycombining is a well-understood concept in many communication systems(including satellite communication systems), we focus here on the typeof diversity combining needed in the above-stated scenario.

SUMMARY

A ground station processes downlink signals received from respectivesatellites. The ground station has a plurality of signal conditioningdevices each receiving a respective one of the downlink signals andproviding a conditioned downlink signal. A plurality of Doppler and/orDelay compensator devices each receive a respective conditioned downlinksignal from a respective one of the plurality of signal conditioningdevices. The compensator devices Doppler and/or Delay compensate thereceived conditioned downlink signal to a nominal zero frequency offsetand a constant delay right through the satellite pass. A selector ordiversity combiner receives the compensated downlink signal from each ofthe plurality of Doppler and/or Delay compensators. The selector ordiversity combiner selects one of the received compensated downlinksignals based on received signal strength of each received compensateddownlink signal to provide a selected downlink signal, or diversitycombines all of the received compensated downlink signals to provide adiversity combined signal. The selector or diversity combiner providesthe selected downlink signal or the diversity combined signal to aneNodeB.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1(a) shows satellites visible to a geographic cell in polar region.

FIG. 1(b) shows the concept of Satellite Diversity Combining (SDC).

FIG. 2(a) is a block diagram of scintillation mitigation throughRSSI-based satellite switching or diversity combining.

FIG. 2(b) shows scintillation/multipath mitigation through RSSI-basedsatellite switching or diversity combining or SIMO.

FIG. 3 is a 2×2 bi-directional MIMO exploiting two-antenna UEs.

DETAILED DESCRIPTION

In describing the illustrative, non-limiting embodiments of thedisclosure illustrated in the drawings, specific terminology will beresorted to for the sake of clarity. However, the disclosure is notintended to be limited to the specific terms so selected, and it is tobe understood that each specific term includes all technical equivalentsthat operate in similar manner to accomplish a similar purpose. Severalembodiments of the disclosure are described for illustrative purposes,it being understood that the disclosure may be embodied in other formsnot specifically shown in the drawings.

Ionospheric scintillations are rapid temporal fluctuations in bothamplitude and phase of trans-ionospheric UHF and L-band signals causedby the scattering due to irregularities in the distribution of electronsencountered along the radio propagation path. The most severescintillations are observed near the poles (at auroral latitudes) andnear the equator (within ±20° of geomagnetic equator). For example, witha polar constellation, satellite selection in polar regions can overcomescintillation loss there, since the terminal devices or stations, e.g.,User Equipment (UE) can see two (or more) satellites 10 a, 10 b servedby the same Ground Station (GS) 200, as shown in FIGS. 1(a), 1(b). Inaddition, depending on satellite elevation seen by the User Equipment(UE), both uplink and downlink encounter near-terrestrial multipathspread.

As further illustrated in FIGS. 1(a), 1(b), UEs such as mobilecellphones are in a ground cell served by a single base station 200,e.g., eNodeB. Those UEs send an uplink signal 14 a, 14 b (which arepower-limited to −7 dBW) in the LTE (Long-Term Evolution) bandsusceptible to scintillation to a respective one of the two (or more)overhead satellites 10 a, 10 b in a low-earth orbit (LEO) constellation.The satellites 10 a, 10 b relay those signals to each of the groundstation 200 (which can be in the UE cell or elsewhere) on a respectivehigh frequency downlink 12 a, 12 b, such as the Q- and/or V-bands. Asionospheric scintillation, when it occurs, diminishes with the square ofthe carrier frequency, the relay downlinks 12 a, 12 b are high enoughfrequency so that ionospheric scintillation is negligible. In addition,the antenna beam-widths to the satellites 10 a, 10 b from the GS 200 arenarrow enough to avoid near-terrestrial multipath.

Turning to FIG. 2 a , a first and second ground station 200 a, 200 b areshown, each having a respective first and second antenna 202 a, 202 b,first and second Low Noise Block Down Converter (LNBC) 204 a, 204 b,first and second Doppler/Delay Compensator 206 a, 206 b, aSelector/Diversity Combiner 208, and a processing device, e.g., eNodeB210, for a given cell. The antennas 202 each receive/transmit ainbound/outbound signal from/to a respective satellite 10, 20 and theground station. The received downlink signals 12 a, 12 b are received bya respective one of the antenna 202 a, 202 b, then provided to arespective signal conditioning device 204 a, 204 b. Thus, the firstantenna 202 a receives the first downlink signal 12 a and provides thatfirst downlink signal 12 a to the first signal conditioning device 204a; and the second antenna 202 b receives the second downlink signal 12 band provides that second downlink signal 12 b to the second signalconditioning device 204 b.

A separate signal conditioner 202 a, 202 b is provided for each downlinksignal 12 a, 12 b from a respective satellite 10, 20. The signalconditioning device 204 a, 204 b translates signal frequency from Q-bandto the LTE-band, filters out-of-band spurious, and amplifies it prior tofurther processing, to provide a conditioned downlink signal 15 a, 15 b.

In addition, a separate Doppler/Delay Compensator 206 a, 206 b isprovided for each downlink path. Each of the conditioned downlinksignals 15 a, 15 b from the respective satellites 10 a, 10 b arereceived by a respective Doppler/Delay Compensator 206 a, 206 b. TheDoppler/Delay compensator 206 a, 206 b eliminates Doppler andnominalizes delay if a UE is located at the center of the cell. At otherUE locations, Doppler and Delay are nearly compensated (to within 0.1 msand to within 600 Hz at cell edge at the highest UE to eNodeB frequencyconsidered, i.e., 900 MHz). The Doppler/Delay Compensator 206 a, 206 bthen outputs a respective compensated downlink signal 16 a, 16 b.

The satellite ephemeris input to each compensator 206 a, 206 b is thetrace of the respective satellite which is a typical two-line element(TLE) data readily available (e.g., on the internet). The ephemeris dataindicates the position and orbital parameters of the satellite. This canbe used, for example, to compute the radial velocity component (andhence Doppler) from the ground station.

The ground station location information input to both of thecompensators 206 a, 206 b can be, for example, from the GPS location ofthe ground station 200 a, 200 b. The ground cell location informationinput to both the compensators can also be from a local database thatlists the cells to which the eNodeBs at GS are providing the service.There is usually a small residual frequency and phase mismatch in thecompensators 206 due to either the TLE and/or the ionospheric delayand/or the terrestrial multipath environment (e.g., when the direct pathfrom the UE to the satellite is blocked). This mismatch determines thetype of diversity combining used. Both the satellite ephemeris and theground station data are used as the radial velocity component (henceDoppler) so that both delay and Doppler can be compensated by thecompensators 206.

In the downlink path, the compensated downlink signal 16 a, 16 b that isoutput from each of the compensators 206 a, 206 b is received at theSelector/Combiner 208. The Selector/Combiner 208 performs signalselection based on a Receive-Strength Signal Indicator (RSSI) (e.g., theRSSI determines the strength of each signal 16 a, 16 b, and the selector208 selects the compensated downlink signal 12 a, 12 b with the greaterstrength) or can perform Diversity Combining. Since diversity combiningsignals requires residual frequency and phase mismatch from thecompensators 206 to be near-zero, RSSI-based signal selection isutilized, rather than diversity combining. Thus, in most cases, theSelector/Diversity Combiner 108 will select one of the compensatedsignals from one of the compensators 206 and output a selected downlinksignal (either the first compensated downlink signal 12 a or the secondcompensated downlink signal 12 b) for transmission to the eNodeB 210that serves that cell.

Alternatively, in cases where the two compensated downlink signals 16 a,16 b have a near-exact compensation (e.g., such that a stronger signalis not readily apparent), the two compensated downlink signals 16 a, 16b are instead diversity combined (e.g., coherently combining the twosignals). For example, when both signals levels are within a threshold(e.g., 3 dB, though other suitable thresholds less than 3 dB can beused) of each other, the selector 208 does not select one of the signals16 a, 16 b, but instead the signals 16 a, 16 b are diversity combinedand presented to the eNodeB 210. In this manner, we improve availabilityand reliability of high average-revenue-per-user (ARPU) subscribers inchosen locations. For example, diversity combining can be utilized whenthe two signals can be made coherent (i.e., with the same frequency andphase, thus adding signals after the phase shift between a commoncomponent in them is made close to 0) (e.g., when the signals havenear-equal levels). Accordingly, the Selector/Combiner 208 eitherselects one of the two compensated downlink signals 16 a, 16 b (if oneis stronger than the other by more than the threshold), or diversitycombines the two downlink signals 16 a, 16 b, which can be done inaccordance with any suitable technique for example by coherentlycombining the two compensated downlink signals 16 a, 16 b.

Thus, at the Ground Station 200, both Doppler and delay in the twosatellite paths (from a single cell) are compensated by the compensator206 a, 206 b as to nearly seamlessly switchover (the stronger signal isswitched in using receive signal strength indicators, or RSSIs) from onesatellite path to the other (or to diversity combine one satellite pathwith the other). The switchover can occur in real time. And while twopaths are shown in the figures, more paths can be provided and thecombiner 208 can combine or select from amongst all of the paths.

The effect of the RSSI-based switching mechanism is seen as morefrequent LEO satellite hand-offs—break-before-makes—from a fixed groundlocation. This is because scintillation loss mitigation is based onmeasured signal strength (RSSI), rather than only satellite ephemeris,where Doppler and delay compensation must continuously operate on atleast two satellite paths (in convention systems, Doppler and delaycompensation at the ground station, GS, occurred for only theephemeris-based selected satellite).

Because resource block durations are short in extant mobilecommunications protocols, diversity combining may not be necessary.However, with two (or more) antenna eNodeBs 210 a, 210 b, diversitycombining inherent to single-input multiple-output (SIMO) may beexploited (after Doppler and delay compensation as shown in the firstprocessing device 210 a option of FIG. 2 b ).

When ionospheric scintillation occurs, one or the other satellite pathon the downlink signal 12 a, 12 b is less affected by it. In this case,the RSSI-based selection at the selector 208 provides the less affectedsignal (i.e., the stronger signal 16 a, 16 b). In other cases, whenionospheric scintillation is minimal, the signals 16 a, 16 b are withina threshold of each other, and coherent combining provides a better SNRsignal to the eNodeB 210.

The signal conditioning devices 204 receives the forward link signalfrom the eNodeB 210, translates signal frequency from LTE-band toQ-/V-band, amplifies it and transmits it to the respective satellite 10,20 using the appropriate gateway antenna 202.

Although the forward link (eNodeB 210 to the UEs, see FIG. 3 ), is alsoin a similar frequency band as the return link (UEs to the eNodeB, seeFIGS. 2 a and 2 b ), and has similar losses (for satellite selection),the forward link satellite selection is only ephemeris-based (ratherthan RSSI-based) primarily because beam generation is from either thefirst satellite 10 a or the second satellite 10 b to the cell (unlikethe reverse link where both satellites 10 a, 10 b are active—for thegiven cell—and RSSI-based switching to the eNodeB occurs at the GS 200).It is noted that if both satellites 10 a, 10 b are active on the forwardlink, the two signals could interfere at the UE (and the eNodeB 210prevents that by selecting one or the other satellite path 206 a/204 aor 206 b/204 b). Choosing the satellite which has a path with thehighest average ground elevation is one selection method, though theremay be other criteria.

Referring to FIG. 2(b), the ground station system 200 can be configuredto have a first processing device (e.g., the first eNodeB 210 a) and asecond processing device (e.g., the second eNodeB 210 b). The UEs couldbe configured for 2×1 MISO (multiple-input, single-output) 210 a, sothat both satellites paths can carry the same down-link signal. Thefirst eNodeB 210 a receives the compensated downlink signals 16 a, 16 bfrom the compensators 206 a, 206 b in the 2×1 MISO. At the same time, asecond eNodeB 210 b can be a 1×1 SISO (single-input single-output). Theselector/combiner 208 receives the compensated downlink signals 16 a, 16b from the compensators 206 a, 206 b and sends its output (either thefirst compensated downlink signal 16 a or the second compensateddownlink signal 16 b; or the diversity combined first and secondcompensated downlink signals 16 a, 16 b) to the second eNodeB 210 b.When UEs operate in 1×2 SIMO mode, the downlink signals 12 a, 12 breceived from both satellites 10 a, 10 b are different. Hence, we cannotuse the diversity combiner 208. The diversity combiner 208 is onlyapplicable in SISO mode, when identical signal comes through differentsatellite channels. FIG. 2(b) when operating in SISO mode is same asFIG. 2(a), and the compensated downlink signals 16 a, 16 b are processedto the second eNodeB 210 b via the selector/diversity combiner 208. Whenoperating in MISO mode, FIG. 2 b further shows the additional option ofhaving a 2×1 MISO eNodeB 210 a when compared to FIG. 2 a , and thedownlink signals 16 a, 16 b are processed by the first eNodeB 210 a.

Turning to FIG. 3 , another embodiment of the disclosure is shown forthe forward link signal, where the ground station 200 has a full 2×2MIMO (multiple-input multiple-output) processing device 210 (e.g.,eNodeB). The eNodeB 210 can be enabled for dual-antenna (in general,multiple antenna) UEs and eNodeBs. MIMO is limited, in addition to thenumber of antennas the UEs and eNodeBs support, by the number ofsatellites in view from a UE and the number of antennas at the GS 200.As shown, a separate signal conditioning device and Doppler/Delay device206 is provided for each forward signal path. However, other suitableembodiments can be utilized. For example, the signal conditioning andDoppler/Delay can be provided in a single integrated unit, which canalso include a selector/diversity combiner. For example, the signalconditioning, Doppler/Delay, and selection/diversity combining can beperformed at or by the eNodeB. In addition, while two downlink signalpaths are shown (each from a respective satellite), more than twodownlink signals can be accommodated, each one having a respectivesignal conditioning device and Doppler/Delay device. In otherembodiments, the conditioning device can be optional and need not beprovided.

In one embodiment, one or more of the components, such as the eNodeB210, Selector/Combiner 208, compensators 206 a, 206 b, and/or theconditioning devices 204 a, 204 b, can be performed by or include aprocessing device, without any manual interaction. All of the componentscan be performed by a single processing device, or by separaterespective processing devices. In addition, two or more of thecomponents can share a processing device, e.g., the first and secondconditioning devices 204 a, 204 b can be implemented by a firstprocessing device, and the first and second compensators 206 a, 206 bcan be implemented by a second processing device. Or, the firstconditioning device 204 a and first compensator 206 a can be implementedby a first processing device, and the second conditioning device 204 band second compensator 206 b can be implemented by a second processingdevice. The processing device can be a processor, computer, server,microprocessor, controller, smartphone or the like.

The foregoing description and drawings should be considered asillustrative only of the principles of the disclosure, which may beconfigured in a variety of ways and is not intended to be limited by theembodiment herein described. Numerous applications of the disclosurewill readily occur to those skilled in the art. Therefore, it is notdesired to limit the disclosure to the specific examples disclosed orthe exact construction and operation shown and described. Rather, allsuitable modifications and equivalents may be resorted to, fallingwithin the scope of the disclosure.

1. A ground station for processing downlink signals received fromrespective satellites, said ground station comprising: a plurality ofsignal conditioning devices each receiving a respective one of thedownlink signals and providing a conditioned downlink signal; aplurality of Doppler and/or Delay compensator devices each receiving arespective conditioned downlink signal from a respective one of theplurality of signal conditioning devices, conducting Doppler and/orDelay compensation on the received conditioned downlink signal, andproviding a compensated downlink signal output; and a selector ordiversity combiner receiving the compensated downlink signal from eachof said plurality of Doppler and/or Delay compensators, said selector ordiversity combiner selecting one of the received compensated downlinksignals based on received signal strength of each received compensateddownlink signal to provide a selected downlink signal, or diversitycombining all of the received compensated downlink signals to provide adiversity combined signal, and providing the selected downlink signal orthe diversity combined signal to an eNodeB.
 2. The ground station ofclaim 1, wherein the downlink signals are high frequency signals.
 3. Theground station of claim 1, wherein the downlink signals are in the Q- orV-band.
 4. The ground station of claim 1, wherein the signals betweenthe ground station and the satellite are sufficiently high frequency sothat they are unaffected by ionospheric scintillation.
 5. The groundstation of claim 1, wherein the downlink signal is relayed from asatellite.
 6. The ground station of claim 5, wherein the satellitereceives an uplink signal from a terminal device in LTE.
 7. Acommunication method comprising: receiving at a ground station, ahigh-frequency downlink signal from a plurality of satellites;conditioning the received high-frequency downlink signals from theplurality of satellites; compensating the conditioned downlink signalsfor Doppler and/or delay; selecting one of the conditioned downlinksignals based on received signal strength to provide a selected downlinksignal, or diversity combining the downlink signals to provide adiversity combined signal; and providing the selected downlink signal orthe diversity combined signal to a base station.
 8. The communicationmethod of claim 7, further comprising: transmitting by a terminaldevice, an LTE uplink signal to a satellite; and relaying by thesatellite, the uplink signal as the high-frequency downlink signal.